The present invention relates generally to analyzing digital imaging, and more particularly to computer aided detection (CAD) of abnormalities in a three dimensional (3D) mammography method, system, and apparatus.
It is well know in the medical community that breast cancer is a leading cause of death in women (and to a lesser extent also affects men). When early detection of abnormalities is combined with proper medical attention, however, the risk of death and/or severe medical ramifications can be drastically reduced. Many devices and techniques (e.g., mammography) are currently under development to detect breast abnormalities earlier, and with greater accuracy than conventional devices. A brief summary of some conventional devices, techniques, and their limitations follows.
Presently, the vast majority of mammography devices utilize conventional x-ray techniques. A patient's breast is positioned in an x-ray machine, which passes x-rays through the breast and generates a corresponding x-ray image on a film. The film is then analyzed by a trained clinician, who examines the film for abnormalities, such as a mass, a cyst, a microcalcification, a fibrous finding, an architectural distortion, and/or other abnormal findings related with benign or malignant abnormalities. In standard digital mammography, the x-ray image (or projection radiograph) is acquired by means of a digital detector, and the resulting digital image can be processed to enhance the visibility of structures within the image, thus providing a potentially more useful image to the clinician. These standard mammography techniques, however, suffer from many problems.
One problem with film based mammography can be generally referred to as film saturation. To fully penetrate through dense parts of the breast, a higher dose of radiation is utilized, generally on the order of about 3 Gy. In relatively dense parts of the breast, a sizeable amount of the radiation is absorbed by the dense tissue, the residual radiation exposing the film. Due to the large x-ray absorption within the dense tissue, the film is not saturated by too much residual radiation, and thus provides sufficient contrast for detecting abnormalities. Near the edges of the breast (e.g. near the skin surface), however, the higher dose of radiation is absorbed to a lesser extent, thus a higher amount of residual radiation exposes the film, which results in film saturation. Film saturation can lead to lower contrast (if any at all) especially near the edges of the breast, and may hinder the clinician's ability to properly identify an abnormality.
Furthermore, the 2D nature of standard mammography techniques (including standard digital and film based) also leads to superposition (e.g., overlay) problems. Superposition can occur when multiple structures are overlaid onto the same position in the projection image. The overlaid normal (i.e., non-malignant) structures may end up combining in appearance to appear as an abnormality, resulting in a “false positive” identification of an abnormality. Presently, the false positive rate is relatively high: on the order of between 70% and 90% of biopsies are normal. Conversely, real abnormalities may be superimposed over dense tissue regions which “hide” the abnormality within the dense tissue, resulting in a “false negative” miss of an abnormality. Thus, in standard 2D imaging (e.g., projection radiography) structures within the breast may become superimposed with each other, thereby normal structures within the breast can “interfere” with a clear interpretation of structures of interest (e.g., potentially malignant) which are located at a different height (relative to the projection direction) within the imaged object.
Another problem with many mammography techniques is related to contrast and structural orientation issues. Radiation passing through the breast is used to generate a view of the breast. “Image slices” of the breast are then generated from multiple views using conventional or newly developed algorithms. As used herein “image slice” is a single image representative of the structures within an imaged object (e.g., breast tissue) at a fixed height above the detector. Abnormalities having a substantial size in the direction approximately parallel to the detector surface will thus generally appear in the image with sufficient contrast and size to be detected by a trained clinician. Abnormalities having a relatively small size in the direction approximately parallel to the detector surface (e.g., a thin duct running substantially perpendicular to the detector surface), however, may only appear as a very small dot in the image. The “dot” like appearance of abnormalities that do not run substantially parallel to the detector surface may hinder the clinician's ability to properly identify an abnormality.
Another problem with conventional mammography techniques is directly related to the importance of having trained and experienced clinicians examining the image (e.g., the film). Without proper training (or even through inadvertence of a trained clinician), abnormalities may be missed, especially when they are relatively small or low contrast in appearance. Moreover, even a well trained clinician generally will not always be able to fully analyze the image in view of previous mammograms and/or patient history (e.g., family history, prior mammograms, health history, lifestyle history, etc.) due to time considerations, fatigue, etc., such that the clinician may not always catch a progression of tissue growth or tissue changes that would be more apparent when considering the additional information.
On inspection of mammograms by a clinician, sometimes radiologists identify suspicious regions (e.g., abnormalities) and request follow up examinations of the breast with ultrasound, nuclear medicine and/or further diagnostic x-rays. The follow up ultrasound and/or nuclear medicine examinations, however, are generally conducted on an entirely different machine than the mammography device, these machines commonly having an entirely different patient configuration and/or image acquisition geometry for different modalities. It is thus difficult (if even possible) to spatially correlate image acquisitions from other modalities with the mammograms. Thus, there is some uncertainty as to whether the follow up scan locates and characterizes the same region. Indeed, it has been estimated that at least 10% of the masses that were identified on ultrasound scans as corresponding to the mammographically suspicious regions were found to correspond to very different regions in the breast. In fact, this percentage is expected to be significantly higher in patients with dense breasts.
Thus, a need exists for an improved method and apparatus for the detection of abnormalities within tissue.